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Re aircraft design

07/18/2009 11:04 PM

Why is design of aircraft limited to single wing design, when two wings would increase lift ?

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#1

Re: Re aircraft design

07/18/2009 11:08 PM

Have you considered the additional non lift generating mass of structural members to support the additional Wings?

As my old professor used to say"Please show calculations."

At some point you get diminshing returns.

milo

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#2

Re: Re aircraft design

07/19/2009 12:18 AM

The short answer is, "it wasn't".

But experience has shown that the extra drag and weight of any rigging between then offsets any advantage. And one (major I think) additional factor you may not be aware of is that unless the wing separation is very large (maybe several times the chord of a single wing) one or both wings have a negative aerodynamic effect on the other.

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#13
In reply to #2

Re: Re aircraft design

07/20/2009 1:07 AM

"one or both wings have a negative aerodynamic effect on the other."

but in helicopter blades (and propellers), this same effect does not limite the helicopter rotor from providing lift, so I don't think it is a show stopper.. but really just requires a higher power to weight ratio if you employ it.

Chris.

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#25
In reply to #13

Re: Re aircraft design

07/20/2009 6:23 AM

Is it because the blades of a rotor/prop are in the same plane vs. multiwing. In other words the other wings may diminish effect the lift capacity of the lower? Don't know, just surmising

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#26
In reply to #13

Re: Re aircraft design

07/20/2009 8:19 AM

I only took 1 gas dynamics class in college but don't helicopter blades serve a completely different purpose?

I thought helicopter create lift by creating a downward airflow so that the force of airflow exceeds the weight of the helicopter (similar to the airflow created by a ceiling fan). Aircraft wings work on a pressure differential between the top and bottom of the wing to create lift.

Helicopter: f=ρAVĀ²

Aircraft: L=(LowerPressure-UpperPressure)(chord)*cos (angle of attack)

Correct me if I'm wrong on this though because I've always found this stuff interesting.

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#32
In reply to #26

Re: Re aircraft design

07/20/2009 11:00 AM

I think you are mostly right, but if it were just a matter of moving air, wouldn't you want to use something more like a turbine blade? Why do they bother shaping the helicopter blades into a foil shape instead of a curved shape like a turbine? I think that the pressure difference still has a role to play. Yes the air is moving, (especially when moving forward) so the tip plot is like a helix, so there is less turbulence on each blade. Also if you look at the image below, you will see vortices trailing the rotor. does this not imply a total lift differential?

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#35
In reply to #26

Re: Re aircraft design

07/20/2009 11:30 AM

Both wings and rotors have to do both - pressure difference between top and bottom provides lift, and the amount of lift applied to the aircraft is exactly equal to the total downward momentum applied to the air in a unit of time. There's no way this can be avoided unless you have ground effect.

The reason they look so different is that, because of the large area and forward motion, a standard aeroplane wing can displace such a large mass of air that the downward velocity can be (relatively) small compared with the plane's forward motion. The relatively small volume of moved air (and high velocity) is also the reason that helicopters use so much fuel.

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#57
In reply to #35

Re: Re aircraft design

07/23/2009 8:26 PM

"pressure difference between top and bottom provides lift"

Bernoulli's principle - Wikipedia, the free encyclopedia

I read somewhere, in a credible publication, as I recall, that the true reason planes can generate enough lift to fly is the downward rush of air as it passes over the rear of the wing.

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#60
In reply to #57

Re: Re aircraft design

07/24/2009 7:24 AM

This pressure difference was not intended as an explanation of why things happened, and neither can "the downwards rush ... over the rear of the wing" be used for that purpose. Both are effects, not reasons.

The basic reason is momentum. If we could ignore interactions between air molecules we could consider air as bouncing off the bottom of the wing, and this model becomes almost credible for ultra-high-altitude hypersonic flight. Otherwise, even the most basic complexities become horrendous.

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#79
In reply to #35

Re: Re aircraft design

11/26/2009 10:01 PM

I have VTOL design. This design has one engine and no exterior moving parts. This design should keep craft stable in flight. Looking for expert to look at drawing.

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#63
In reply to #2

Re: Re aircraft design

07/24/2009 10:39 AM

With the use of aluminum and plastics they can build a bi-plane without out the added rigging.

They still use bi-planes for crop dusting. They are useful for slow flying and added maneuverabilty in avoiding power lines.

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#3

Re: Re aircraft design

07/19/2009 12:30 AM

see Pitts Special.

imho :

but in the day of Fokker and Sopwith, the power plants weren't as reliable.

the introduction of the radial engine helped. then of course jet's came along.

simple, yes, also the crossing of the speed of sound, don't think bi or tri wing could accomplish that feat.

straight wings vs swept wings.. compressibility, newer materials, etc.

it's easy in todays environment to think of speed. but the 100 mph barrier was a substantial one 100 years ago.

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#4

Re: Re aircraft design

07/19/2009 1:33 AM

If bi, tri or quad wings are better why :-

1) were birds not created that way?

2) did birds not evolve that way?

strike out 1 or 2 according to your preference.

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#14
In reply to #4

Re: Re aircraft design

07/20/2009 1:09 AM

There are many other flying creatures that do employ multiple wings. I do believe that birds use single wings, as the most efficient for the power, but I don't think that we are limited to that for our purposes. There are instances in design where multiwings can be used to maximize lift, either for heavier loads, or narrower widths.

Chris

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#17
In reply to #14

Re: Re aircraft design

07/20/2009 1:54 AM

I realized that soon after posting.

I like your improved box kite design.

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#23
In reply to #14

Re: Re aircraft design

07/20/2009 5:28 AM

This thread is interesting for several reasons. The postulation re the multiplicity and shape of the foils, added to the disussion in operational envelope is provoking.

Think of this.

About 50 years ago, as kids, with no scientific education whatsoever, a group of us mused on the boomerang. Made many non-airfoil ones out of plywood scraps. Had tremendous fun beheading all sorts of things on the farm (mainly scotch thistles).

Wondered if we could make an airplane.

Took a foot by one-an-a-half feet rectangle of 3/32 ply and cut an engine mounting notch in the short, 'leading' edge, and bolted a 5cc motor to it. Behind it made a rough 'fuselage' to house (which progressed into a big fin) the control servos, which were conneced to two 'elevators', on the rear edges ( just some full width pieces of the same ply, cut in half to accomodate left and right) and a rudder. A rudimentary cart (taildragger) and it was complete.

Flew like an eagle, and not a airfoil in sight. Well, maybe 'eagle' is not the real word. Unstable to hell. Took ages to master it, eventually, rarely used the rudder. Climbing turns.

Lesson? Given enough power you can make a housebrick fly.

Sorry! Just thought you'd be interested.

Cheers,

Stu.

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#24
In reply to #23

Re: Re aircraft design

07/20/2009 6:00 AM

Hi Stu.

Is that not just the best! As a kid I too just loved making things fly. Call them things for good reason. Later things did progress toward aircraft status and the fun level seemed to drop with the inverse proportion of cost!

I had to make and sell about a 120 hot dogs a week at boarding school to keep my aeronautical capers financed>>>> damm if that was all it would cost today!!

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#43
In reply to #23

Re: Re aircraft design

07/21/2009 4:07 AM

... sounds good to me

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#45
In reply to #23

Re: Re aircraft design

07/21/2009 10:05 AM

McDonnell Douglas did something similar in the 1950's. They called it the F-4 Phantom II. To quote a college professor who was on the test team "It was big. It was fast. With those two big, booming engines, they could make a Dempsey Dumpster supersonic....Come to think of it, they did."

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#46
In reply to #45

Re: Re aircraft design

07/21/2009 10:52 AM

or they could fly 40 knots,.

or they could take a missle thru the wing & still fly home, or they could...

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#33
In reply to #14

Re: Re aircraft design

07/20/2009 11:02 AM

Well if birds were to have four wings then that would imply that humans could have four arms.

Any if a bird were to have four wings that would make the bird an insect.

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#19
In reply to #4

Re: Re aircraft design

07/20/2009 2:26 AM

imho :

birds have a shoulder socket...

seen movies of the early pioneers?

tried to be bird men, stacked up 12 wings , did lots....

no one single airplane design fits all. cargo , pax, stunt , all have special needs, one size does not fit all , folks.

some of the biggest fiasco's are airplanes that tried to do too much , and couldn't do anything well enough. & that is the point.

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#47
In reply to #4

Re: Re aircraft design

07/21/2009 5:11 PM

Excellent job putting them both out there .

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#5

Re: Re aircraft design

07/19/2009 12:59 PM

Saying in other words, why should you include an additional pair of wings, if one is able to handle the job? More structure, more corrosion spots, more paint, more control surfaces... In aviation, if it's working, keep it as is. Any addition would only increase weight and problems.

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#6
In reply to #5

Re: Re aircraft design

07/19/2009 2:15 PM

A most interesting answer. All answers good so far.

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#7

Re: Re aircraft design

07/19/2009 2:19 PM

Further to this question; How is lift generated ? Versions differ, with some referring to the "vacuum" above the wings, to mass air flow forced down by wings, thus generating lift. Are bot forces not acting ?

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#41
In reply to #7

Re: Re aircraft design

07/20/2009 5:17 PM

Air moving over the top of the wing has more velocity than the air moving under the wing because the air isn't separated. The air is staying in place while an object moves through it. Therefore, there is less pressure (not a vacuum) over the top of the wing. When the pressure differential between the top and bottom of the wing factored by the wing area exceeds the weight of the aircraft, you have sufficient lift to fly.

Propellers and helicopter rotors (also called rotating wings) work by the same principle; pressure differential.

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#8

Re: Re aircraft design

07/19/2009 10:50 PM

Actually, there are aircrafts designed with two wings, and those mainly used in armies for heavy loads. But, if one wing is capable to do the lifting, why you need more. And I think that aircrafts furnished with only one wing has more capability for manoeuvring.

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#9

Re: Re aircraft design

07/19/2009 11:57 PM

Additional wings add drag and mass to the structure. Multiple wings were necessary in the "old days" in aviation's infancy, but engineering advances have made them redundant.

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#10

Re: Re aircraft design

07/20/2009 12:25 AM

In a word, drag. All those struts and extra intersections create loads of drag. High aspect ratio monoplanes give the best efficiency - at the expense of structural weight.

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#11

Re: Re aircraft design

07/20/2009 1:03 AM

I think there might be benefits to multiwing approaches.. thats why I came up with this idea.

and for anyone interested in other roadable aircraft, check this site.

Chris

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#15
In reply to #11

Re: Re aircraft design

07/20/2009 1:20 AM

That's impressive.

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#16
In reply to #15

Re: Re aircraft design

07/20/2009 1:50 AM

I wrote to the gentleman who runs that site, and he said he would add my design to the list, and wanted any other designs I came across, to notify him... He won't get to update the site for another month or two, so I'm hoping to refine the design by then.

thank you

Chris

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#31
In reply to #11

Re: Re aircraft design

07/20/2009 10:51 AM

Here is another aircraft I just found, which is obviously functional. I see similarities to my design.

http://billskestrelhawk.com/aboutus.html

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#39
In reply to #31

Re: Re aircraft design

07/20/2009 3:06 PM

Here is another aircraft (and site) I just found, with what they call a 'tandem wing' configuration.

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#75
In reply to #39

Re: Re aircraft design

07/30/2009 10:33 AM

Chris, there is a definite advantage to the extra lift v.s. drag and weight with a float plane. Both for low speed landing and getting the floats out of the water.

Great pics.....

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#76
In reply to #75

Re: Re aircraft design

07/30/2009 11:24 AM

Presumably the the ground effect is really good at take-off? And the extra drag of the floats mean that this is not going to fly so fast that the wings end up interfering with each-other?

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#77
In reply to #76

Re: Re aircraft design

07/31/2009 12:54 PM

The plane pictured in #39 is the famous "Flying Flea" (pou de ciel, if I recall) which has been around for decades and seems always to be built the same, even to the profile of the rudder. It has a mixed reputation, but the advocates for the design claim is is very safe and "stall proof." (Some claim it is unsafe) The rear wing flies in the downwash of the forward wing and, given that the forward wing should stall first, while the rear wing is still lifting, the stall should be gentle. The relative position of the wings and their angular relationship seem to be critical. Other tandem-wing planes have been built (recently some Burt Rutan designs and the "Quicky" homebuilt), but I can't think of any true tandems which were ever mass produced. As I recall, there was once a giant seaplane with three sets of biplane wings strung down the fuselage. It's not clear that it actually flew, and if it did it was obviously not imitated.

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#12

Re: Re aircraft design

07/20/2009 1:03 AM

2 wings also increase drag & limit speed. crop dusters use bi-planes as they do not need to go fast but need to carry heavy loads of pesticides to drop. high speed jets have minimum wins to lower drag & increase speed without shredding themselves in the air. drag adds load to airframes.

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#28
In reply to #12

Re: Re aircraft design

07/20/2009 9:19 AM

Actually, few cropdusters (more properly, known today as agricultural aircraft) today are biplanes. Most, if not all still in production are monoplanes having fairly high aspect ratios. The longer wing increases fuel efficiency while providing a greater swath width for each pass.

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#18

Re: Re aircraft design

07/20/2009 2:08 AM

To put two wing together and NOT have drag you need to avoid a throttle (converge/diverge) effect. Most wings have a curve to them...and when you put two curves together you get...a throttle...

So, in order to put two wing together and get low drag the bottom wing's top surface needs to be rather flat and the top wing's lower surface needs to also be flat. A flat topped airfoil (rectangular) will not do well in a stall...See the problem?

That said, I worked on a bi-wing and made it very low drag and high lift...the bottom wing was NOT rectangular...

If you want to stack them vertically, the problems of drag multiply... Spacing them longitudinally helps...

The real question them becomes...what airfoil shaped to use. The third foil of three, for example, may be much different than the middle foil, or the bottom of three...or in a stack of four...etc... Remember, a single foil works by itself (monowing) and the shape will give a different effect when stacked in groups.

Basically, this requires working out custom airfoils and requires new airfoil development, which most people who design airplanes try to avoid.

Also, a multiple wings vertically work on the same air and induce drag into each other and lower the amount of momentum each wing can produce.

Seaplaneguy.

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#21
In reply to #18

Re: Re aircraft design

07/20/2009 4:17 AM

GA.

Your answer appeared to be the best and most accurate that I have read when reading from the top down, but I am still a "Layman" in aerofoil terms!!!

Yours seem to make good analytical sense.....thanks.

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#29
In reply to #18

Re: Re aircraft design

07/20/2009 9:55 AM

Correct me if I'm wrong but from what I've been reading. The more wings you have the more lift you have, with the right wind design of course. But because of the high drag they create you can't go fast. So what if you had something, a platform maybe, that you didn't what to go very fast but you had a lot of weight you were lifting. Would multiple wings be able to create lots of lift with a minimum amount of HP required for that lift.

The reason for my question is that I've always thought there might be a market for something like a floating cruise ship. You could take this places a cruise ship could only dream of. You could follow the Amazon in prefect comfort. Have small planes for day trips. It's a fun idea.

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#30
In reply to #29

Re: Re aircraft design

07/20/2009 10:46 AM

like this?

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#34
In reply to #30

Re: Re aircraft design

07/20/2009 11:10 AM

That things cool. But there isn't much chance I'd be able to afford a ticket. :-) But I was thinking more along the lines of something like that but with 50 wings sticking out each side maybe even front and back. The wings would be stacked like bi-plane wings but with a lot more layers. All those wing would give you your lift. It would be slow and ugly as sin. Kind of like all those wings you've got on your car design. I like your car/plane design by the way. Keep on designing, we need more people that can think outside the box. And that design of yours is outside the box.

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#20

Re: Re aircraft design

07/20/2009 3:24 AM

Because the structures needed to support the second wing also increase drag.

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#38
In reply to #20

Re: Re aircraft design

07/20/2009 2:26 PM

thank you.

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#22

Re: Re aircraft design

07/20/2009 5:22 AM

Good day to all the good people! Nobler you got some good answers from the first by Milo: Do the sums! and every answer thereafter is about doing the sums. The challenge with aircraft is not designing them, it's putting your own but on one of the seats when it takes off. Oh and then remembering that one little sum........

Trevorm indirectly points out that there are many multi-wings. Well the conventional is in fact a two winger, main and horizontal stabiliser is also a wing, in many cases providing negative lift to control the torsional moment created by the main wing. In canard form, small wing in front and big at back, both usually provide lift. That can result in lowers lift/drag ratios...do the sum! Burt Rutan is likely the best known and I venture (please don't beat me if I'm wrong) most successful canard designer of our times. There are single wing planes, like the old Davis flying wing (no tail) and many more and your delta wing mill jets.

Multi wings (including our conventional planes, main and tail) all have a challenge, and this is where Chrisg288 has made lots and lots of sums for himself. (Great idea and inspiration there Chris) Airflow is simple in one set of criteria, i.e. one speed and direction with all controls neutral (if I may). You can play with positions of wings to get the least interference from each other. (think F1 race cars in clean air and loss of down-force in dirty air behind another car) When the plane changes its attitude (overall direction) to the air stream all the surfaces behave differently. In particular a critical lift surface may end up in turbulent air and then.....do the sum! i.e. are there enough parachutes? In a gross generalisation I would say, If a pilot is going to fly it for commercial gain...keep it simple as possible. If a pilot is going to play second fiddle to a giant computer....go against as many laws of physics you have cash to throw at. The computer and a vulgar amout of fuel will make it fly.

Aviation designers love re-doing the sums. The Boeing 747 is a great example. Look up the specs when it first flew and compare them with the last model. To the layman that magnificent lump of ally still looks the same yet thousands of small improvements after millions of sums have allowed it to almost ??double?? its efficiency from when first rolled out.

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#27

Re: Re aircraft design

07/20/2009 8:39 AM

No wing is desired option to decrease drag. Wing is for lift but also a dead weight and decreases efficiency of plan.

In real world senario one wing is comparmize. What one need is no wing plan and wing which is extended or contracted based on need. One wing we utilized to store fuel and came up justification what will be for other to make Saudi's more rish and funnel money to Iran

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#36

Re: Re aircraft design

07/20/2009 11:42 AM

Basically, assuming similar design methods, lift increases more than linearly with the area of a single wing, but drag increases linearly with area. That doesn't mean that multi-wing won't work - but it does mean that a low-loss multi-wing design will be more complex than a single-wing one - so there has to be some other significant advantage.

One example of such an advantage could be profile or maneuverability. Nevertheless, multi-wing does not seem to have been required in recent times.

N.B. Helicopters are a different special case - structural integrity demands a degree of symmetry about the axis of the rotor, so you are stuck with a minimum of two wings scanning the same space. In addition, the relatively high downwards velocity of the air-stream limits the efficiency gains to be made from increasing the area of the individual wing (and possibly also the interaction between the blades).

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#37
In reply to #36

Re: Re aircraft design

07/20/2009 2:24 PM

Beautiful answer. Thanks to you. I am working on a new design, so your comments are valid.

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#40

Re: Re aircraft design

07/20/2009 3:55 PM

Lift= Drag and makes for a slow speed, fuel consuming design, and no aircraft are not limited to single wing design, two wings makes for a great stunt plane at air shows

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#42

Re: Re aircraft design

07/20/2009 9:38 PM

while it is true, 2 wings would increase lift, it will also increase drag. early slow flying airplanes did use two wings ( bi planes ) but modern planes do not need as much lift as they fly much faster, getting the lift from the speed of the air flowing over the wings.

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#44

Re: Re aircraft design

07/21/2009 9:35 AM

with the multi wing design. you get more stability and maneuverability at the slower flight. which in early aviation warfare was critical. plus they did not have the high powered powerplant of today design. if you look at the F-14 it has wing that sweep back at high speed then sweeps forward for more stable control at lower speed. by sweeping forward it increases the lift surface but also the drag. the same for multi wing you have more lifting surface at lower speeds but more drag so top speed is decreased. if you look at bi or tri wing planes their wings are not stacked directly on top of each other. that lot to do the way the air comes of the wings to eliminated the turbulence.

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#48

Re: Re aircraft design

07/21/2009 7:53 PM

Most of the above comments have been missing the point. F=MA, where F is lift, M is the mass of air involved, and A is the acceleration of the air, downward. A thin flat plate (see several jet fighters) at a small angle of attack (basically the angle of the wing to the air flow, whereby it deflects air downward) will do nicely. At zero angle, the deflection of the air (A) is zero, and lift is zero. At one degree, a considerable mass of (essentially incompressible) air is pushed downward, generating lift (remember, F=MA) At two degrees, you get twice the lift, and at three, three times the lift. (Forget that Bernoulli effect crap; it was the fantasy of an uneducated FAA test writer and is disproved as soon as you fly upside down) However, as the downward velocity increases, the energy inparted to the air, induced drag, goes as V^2, while the lift (change in momentum) goes as V, so a smaller V and a larger M is better. Then, at some angle, the air no longer "sticks" to the wing and goes turbulent, a "stall", where too much energy (drag) is being consumed churning the air. If the thin plate is bent into a shallow arc (most WW-1 wing sections), the stall is delayed, allowing more lift when needed for maneuvering.

The problem with thin sections is structural. In the days of wood and fabric wings, one could have one wing (see the Fokker monoplane), strut and wire braced, or two wings which would form a truss, usually with struts in compresson between the upper and lower chords and wires in tension taking the shear loads. The Red Baron, famously flew a triplane (inspired by the British Sopwith triplane), but the beauty of the Fokker triplane was that it used a thick wing section, which allowed a deep spar (cantilever), which allowed getting rid of the bracing wires, which reduced drag. (Apparently early designers assumed small diameter wires did not create much drag, but, like wings, they disturb air on either side and generate a lot of drag) (By the way, many wings, like a Venetian blind, were tried, without much success, and some early rotary-wing aircraft used biplane rotors, for structural reasons) Antony Fokker was a good engineer, and he then built his biplane D-VII with thick wings, deep spars, and no wires. It was probably the best fighter of the war. (The interplane struts were there mostly to reassure the pilot) Since the two wings were no longer parts of a truss, the D-VII could fly with the lower wing removed. The D-VIII had only one wing, but came into service too late to influence the outcome of the war.

So, why is one wing better than two or more? Yes, you don't want struts, although many monoplanes do use them (Piper Cub, Cessna 150). Wing tips are also draggy, as the air curls around and eats energy with vortices. (Note that winglets are showing up on airliners, to reduce the losses at the tip) For efficiency, you want a long wing, so as to influence the largest mass of air, with small tips and only two tips, not four or more. High performance gliders use long narrow wings to get the best available lift/drag ratio, the largest MA for the least MV^2. The faster a plane flies, the more M of air it can influence, per unit time, so fast planes have relatively smaller wing spans for their weight.

Biplanes are still built for various reasons. For example, the Pitts Special is built for aerobatics, so it needs to be strong (biplane truss) with a good roll rate (short wings), and fuel economy is not important (unlike airliners). Actually, if you connect the wing tips of a biplane with a vertical surface, making a box-kite-like structure, you have no wing tips, vice four, and the efficency isn't bad, as long as the wings aren't too close together. Closely spaced wings interfere with each other, as they are both trying to deflect the same large mass of air. (Remember, air is "incompressible", so the air pushed down by the upper wing tends to push down on the lower wing. Ultimately, it pushes down on the surface of the earth, which can be visualized when a plane flies low over water)

Tandem wings, spaced horizontally instead of vertically, have been used, but only in oddball aircraft. If you watch the Fowler flaps of an airliner deploy, moving backward from the main wing, what you are seeing, in effect, is the addition of another wing, in tandem with the main wing. The rear wing, if it is to generate lift, has to accelerate downward the already downward moving air from the forward wing, so the induced drag, the V^2 thing, is increased. Flaps on the trailing edge of wings are useful for landing, as the speed can be reduced (less M) for the same lift (MV is constant) as the V in MV is increased (greater deflection of the air), but the drag increases greatly, again because V^2 is increased. The plane can fly slower, MV being constant, with less M being compensated for by more downward V. Some wings, with elaborate lift-enhancing flaps, begin to resemble a turbine blade(s), but the lift/drag ratio stinks.

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#49
In reply to #48

Re: Re aircraft design

07/21/2009 8:01 PM

All I can say is thank you. GA.

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#50
In reply to #48

Re: Re aircraft design

07/22/2009 2:23 AM

Your wing lift theory is not altogether incorrect, but is not well-supported by either wind tunnel data or CFD.

However, as the downward velocity increases, the energy inparted to the air, induced drag, goes as V^2, while the lift (change in momentum) goes as V, so a smaller V and a larger M is better.

On a wing of infinite span both lift and drag increase with the square of velocity. Therefore, the calculations for the two are identical, with the only difference being that the coefficient of lift is used in one and the coefficient of drag is used in the other.

The flat plate lift theory does not adequately explain why 2/3 of the lift is generated on the top, low pressure side of the wing. In another thread, I provided a hand-drawn but reasonably accurate pressure distribution across the upper and lower surfaces of a wing.

But in other respects, I agree with your post. The more tips, the more tip losses; the more roots, the more root interference; the more structure hanging out in the airflow, the more drag. The inefficiency of a biplane is perhaps better explained by the fact that the high pressure area of the upper wing is located near the low pressure area of the lower wing, with one tending to cancel the other.

Add all this stuff up and the cumulative effects are dramatic: the stubby multiple wings of a biplane operate and 6:1 L/D, and the long thing wings of a sailplane operate a 60:1

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#52
In reply to #50

Re: Re aircraft design

07/22/2009 5:48 AM

"However, as the downward velocity increases, the energy inparted to the air, induced drag, goes as V^2, while the lift (change in momentum) goes as V, so a smaller V and a larger M is better."

You are misreading. While I regard much of what ESB has written as simplistic and probably misleading, the basis for this part is correct. You are presumably writing about fixed wing geometry; ESB here is writing about tailoring the design of the wing to move the air vertically at different speeds. The objective is to show why (within reason) it is a good idea to maximise the amount of air you move vertically and so minimise its vertical velocity; depending on aircraft velocity, that can of course be achieved by appropriate sizing in all three dimensions.

BTW, as you are responding to comments on energy losses, the relevant parameter for a variable wing velocity would be drag x wing-velocity... If we are considering only intrinsic lift-derived drag, the wing shape can be tailored so that vertical air velocity can go as the inverse of aircraft velocity - so the drag force (m.Vvertical2) also varies as the inverse of the aircraft velocity. The competition between this reduction with increasing speed and the increasing turbulent drag is the major reason that every airframe has an optimum economic cruising speed. (Engines come into this as well of course - albeit cruising speed is an input design parameter).

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#78
In reply to #52

Re: Re aircraft design

08/01/2009 1:10 AM

The competition between this reduction with increasing speed and the increasing turbulent drag is the major reason that every airframe has an optimum economic cruising speed. (Engines come into this as well of course - albeit cruising speed is an input design parameter).

great explanation.

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#64
In reply to #50

Re: Re aircraft design

07/24/2009 11:22 AM

I agree with the above (#50), and the drawing of pressure distribution is reasonably accurate, as compared with NACA plots. Well, I question the practicality of a wing of infinite span, a trivial quibble. And yes, both lift and drag increase with the square of the velocity, more or less, at the same angle of attack. If you change the angle of attack, the lift/drag ratio changes. A major reason for that is the V vs. V^2 effect I described, "induced drag" (as distinct from "parasite" drag). If, for instance, you put your craft into a 60 degree bank (holding altitude and airspeed), you will need to increase the the lift, which demands an increase in angle of attack, which will, generally, increase drag proportionately more than lift, if the plane was designed to be most efficient in cruise flight.

(see second historical note below)

Note, however, that the Bernoulli effect depends on the total velocity (speed) of the air, while it is not clear from the drawing that the speed of the air is greatly faster where the pressure is least. If the airfoil shown were exactly symmetrical, instead of nearly so as shown, the pressure distribution would be similar, which kind of disagrees with the assertion that the more curved upper surface is responsible for the lift. (There was a good NACA book, Theory of Wing Sections, if I recall correctly, which has pressure plots for various airfoils at various angles of attack)

Yes, of course the pressure is lower on the upper side. If you are going to accelerate air downward, to generate lift, you need a pressure gradient to move the air, and of course the low pressure "sucks" the wing upward. (And it sucks the air downward) Note that the pressures shown generally correlate with the curvature of the flow, which again is reasonable, since the greater the pressure difference the greater the acceleration, as shown by the curvature, not the speed.

If you look in an FAA manual, trying to explain theory of flight to student pilots, it will show a curved top airfoil (like a Clark Y) with streamlines of air around it. The streamlines at the leading edge and at the trailing edge are horizontal. The assertion is made that the air which parts at the leading edge must meet again at the trailing edge (there is no evidence to support that assertion, of "must"), and since it has a longer distance to go over the top, it must go faster, and Bernoulli says faster air has lower pressure, so the wing is sucked up. However, in reality, if the streamlines at the trailing edge are horizontal (no downward acceleration), there will be no lift; net acceleration is zero. If you integrate the velocity-pressure distribution over the wing, using the assumption that the air at the trailing edge is the same velocity whichever side it passed over, then you find, again, zero lift. The increased pressure behind the high point of curvature, where the air is slowing down, cancels out the lift from the front of the airfoil. Sorry about that.

That is why I characterized the "official FAA" Bernoulli explanation as fantasy.

"Your wing lift theory is not altogether incorrect, but is not well-supported by either wind tunnel data or CFD."

Yes, it's not altogether correct. Viscous effects and such are ignored in the effort to write a simple explanation. Drag is a complex subject, but the fact remains, that, for lift, F=MA, and Bernoulli, as explained by popular science books, does not fit real-world data at all. "My" wing lift theory may have its faults, but it was good enough to design some very good aircraft, many of which are still flying, despite not being "well supported".

A historical note: during WW-1, Albert Einstein tried to contribute by designing a better wing section, with computations based on the Bernoulli principle. It was a failure. Real world data trumps theoretical analysis.

Second historical note, relative to the effect of changing angle of attack: I was once present when an Air Force Colonel was reprimanded for exceeding Mach 3, in violation of his orders, in a YF-12 aircraft ("civilian" version of the SR-71). His excuse was that he couldn't help it; the aircraft kept going faster, even when he tried to slow it by going into a 60 degree bank. The reason that the incresed drag did not slow the aircraft is that the engines at high speed were essentially ram jets, and the thrust increased with speed. In those pre-computer-controls days, attempting to throttle back frequently resulted in an "engine unstart" (polite way to say it stops), and the asymmetric thrust would send the aircraft cartwheeling through the sky like a frisbie. Usually the plane was OK, but the pilot was somewhat jellyfied.

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#65
In reply to #64

Re: Re aircraft design

07/24/2009 12:27 PM

I just scanned this, but think we are on essentially the same page. Such thinking lead me, a while back, to start a (somewhat tongue-in-cheek) thread on the subject. Yes, the classic FAA explanation is incorrect.

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#69
In reply to #64

Re: Re aircraft design

07/24/2009 5:36 PM

Either you simply 'don't get it', or you are happy to waste everyone's time (including your own) rather than admit that you just might have written something misleading.

You wrote "Forget that Bernoulli effect crap; it was the fantasy of an uneducated FAA test writer". You applied the word 'crap' to the Bernoulli effect, not to a specific incorrect explanation or diagram. I've no problem with your rubbishing the carelessly (maybe even ignorantly) written instruction manual. But you dismissed an effect that is truly involved (albeit one that is not obviously helpful in explanation), and apparently you are not prepared to back off on that detail of your writing. So far as I'm concerned, that places you in the same camp of fatuousness as the continued publication of the FAA manual - at least for the time being.

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#51
In reply to #48

Re: Re aircraft design

07/22/2009 3:59 AM

GA from me.

It all made sense and I understood most of it too, So thanks for the good write up as well.....

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#53
In reply to #48

Re: Re aircraft design

07/22/2009 5:51 AM

Much of this is good. But don't believe the bit about Bernoulli being irrelevant; changing the angle of attack mainly works by causing the air to take a longer path past the backside of the wing.

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#58
In reply to #53

Re: Re aircraft design

07/23/2009 9:35 PM

changing the angle of attack mainly works by causing the air to take a longer path past the backside of the wing.

... although certainly not in the sense that the discredited "air meeting up at the back of the wing" theory would support. Changing the angle of attack from 1 degree to 10 degrees on a typical wing creates a 10-fold change in lift, whereas the path length changes only very slightly, and in the discredited model, changes only by the additional path length due to the change in stagnation point on the leading edge. In wings scaled for general aviation aircraft, that stagnation point change is on the order of one inch on a wing with 48 inch chord.

Depending upon aspect ratio, this 10-fold change in lift will also cause more than a 10-fold increase in drag.

It seems more accurate to me to say that changing the angle of attack mainly works by simple flat plate theory: air mass is more vigorously accelerated downward.

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#61
In reply to #58

Re: Re aircraft design

07/24/2009 7:50 AM

Why do you insist on re-interpreting everything as if linked to other stuff that has been discredited - even when that stuff (I hesitate to call it theory) has already been expressly dissociated. Then your so-called counter-example (and the flat-plate theory you cite) operate expressly outside the region of argument (moving more air to improve efficiency) - as evidenced by the drag rising faster than the lift (clearly caused by turbulence that is not directly related to the generation of lift).

Some somewhat disconnected comments:
As you know, the pressure change is generated by relative velocity, not apparent path length per se.
For a wing working in its efficient regime (where the losses are comparable with the downward kinetic energy of the air-stream) the lift on the wing is generated primarily by pressure reduction on the upper surface.

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#62
In reply to #61

Re: Re aircraft design

07/24/2009 7:51 AM

That was accidentally submitted as "guest". It was I

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#72
In reply to #61

Re: Re aircraft design

07/25/2009 1:48 PM

Why do you insist on re-interpreting everything...

Actually I do not insist, although I sometimes persist. And most things that I may be tempted to re-interpret, I simply let slide.

I think ESBuck (who seems pretty savvy) has misinterpreted your statements to mean that you support the over-simplified "theory" still presented in museums, etc as the reason for wings working the way they do. I know that is not the concept you are intending to promote. There are others here who have their own plausible-to-completely-implausible theories on lift and drag generation. Some are so profoundly implausible (such as a contention advanced here recently that wings serve only to provide stability and the ability to turn the aircraft*) that I think it might be good to clarify your position, because if we have people who appear to be struggling with very basic concepts. For those people, your statements could be interpreted as ESBuck seems to have interpreted them. So I offered the clarification.

Admittedly, introduction of the flat plate theory might muddy the waters (water being a place where, for planing surfaces, the simple vector addition approach, based on how much water is pushed forward by the planing surface versus how much is pushed down, gets you quite close to real world results of lift vs drag.)

* Yes, it is true that the work of turning the aircraft is done by the wings, but if their primary function of generating lift with relatively little drag were not fulfilled, they could not work to turn the aircraft.

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#74
In reply to #72

Re: Re aircraft design

07/26/2009 4:21 PM

I think the "purely for turning" are over-interpreting the publicity that went out for the 747 airframe - that most of the lift was provided by the fuselage itself. I never looked into that, but I suppose there are circumstances where that is true?

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#66
In reply to #53

Re: Re aircraft design

07/24/2009 1:15 PM

http://cr4.globalspec.com/thread/2673/Lies-More-Lies-and-Arithmetic

Do the math! Longer path doesn't explain it. This is especially true with undercambered airfoils, like a birds wing or a bent sheet of balsa wood.

The above link is well written and should convince even Physicist.

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#70
In reply to #66

Re: Re aircraft design

07/24/2009 5:52 PM

In most cases the longer path presented by the wing (or even the paths around the wing) does (do) not explain it. But the longer path taken by the air and the consequent velocity (due to recirculation etc) are consistent with the lift generated. The difference between the two velocities seems to be related to the inefficiency of the aerofoil.

If you oversimplify the theory, you miss the effect. But the effect is still there, and Bernoulli isn't contradicted - although (as I've said several times before) it's not usually directly helpful.

BTW, aren't some of the pressure effects in meteorology attributable to recirculation velocities (though the driver in that case is somewhat different)?

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#73
In reply to #70

Re: Re aircraft design

07/26/2009 4:11 PM

P.S. Of course, Bernoulli is based on energy conservation, and poorly-performing aircraft wings (flats, and others in conditions where most of the lift is provided by virtue of the angle of attack) are very wasteful, so (consistency aside) Bernoulli does not explain much under these conditions. And (as previously implied) Bernoulli breaks down completely in the supersonic regime. However, for modest flight velocities Bernoulli becomes more relevant as we reduce drag sources other than those reflected in the downward kinetic energy of the air (but that needs quite large wings).

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#54

Re: Re aircraft design

07/22/2009 3:34 PM

For those of you who beleive in Bernouli:

What makes you think that air divided by the leading edge must necessarily rejoin at the trailing edge? The Bernouli effect works with a venturi, but a wing is not a venturi, and the air flow is not confined. The picture found in FAA manuals and elementary science books, where the airflow at the trailing edge is in the same direction as the airflow in front of the leading edge, is erroneous and physically impossible. There is no force without acceleration (remember Newton?), and there is no net acceleration in the usual description of the Bernouli effect.

Yes, the pressure is less on the upper side of the wing (unless you are flying upside down), as it must be to accelerate air downward, and it is greater on the lower side, as it must be to accelerate the air downward, but there is nothing magic about invoking Bernouli. A symmetric airfoil section works quite nicely, in spite of Bernouli. The point is to deflect the air flow downwar.

Suggest you investigat the Kasper wing. He designed the high lift devices for the Boeing 737, and he found that trapping vortices on the upper side of the wing would enhance the lift coefficient, but, of course, Bernouli was irrelevant. See also the F117 aircraft. The pressure distribution around an airfoil can be described mathematically as a circulation, superimposed on the airflow. If you generate lift, the airflow will be bent downward. If it is not, there is no lift, silly drawings not withstanding.

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#55
In reply to #54

Re: Re aircraft design

07/22/2009 4:34 PM

I assume that was an answer to my post ; if so it would have been clearer if the "reply" button had worked.

A symmetrical wing (top-to-bottom relative to central plane of wing) uses an angle of attack which means its behaviour is not symmetrical. Yes most of the drawings you see are inaccurate as to the flows - and that includes the apparently more plausible ones where the air rejoins at the back of the wing and then progresses tidily at a downwards angle. And I agree that all the other requirements you cite must be met (in fact you will find that I gave these in my earlier analytic posts). None of that means Bernoulli is wrong or irrelevant* - just that it is a part of a more complex picture. You can (as I did previously) analyse parts of the process and their consequences without detailed knowledge of the pressures - but in the end it is pressure differentials that both provide the lift and give the air its downward velocity.

*The complexities of many of the situations mean that the simplifying assumptions (that make Bernoulli a useful analytic tool) break down - so the analysis often proceeds without invoking Bernoulli; nevertheless, the underlying conservative principle is maintained. Wikipedia (the section entitled "Real world application") is quite good on this subject.

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#56
In reply to #55

Re: Re aircraft design

07/22/2009 5:32 PM

Quoted from the cited Wiki article:

"Note that Bernoulli's principle does not explain why the air flows faster past the top of the wing and slower past the under-side. To understand why, it is helpful to understand circulation, the Kutta condition and the Kutta-Joukowski theorem. Another way to explain why the air speeds up is to observe that the air is deflected downward in the vicinity of the wing. Newton's 1st & 3rd law imply that there is an upward force on the wing, which implies an area of low pressure on the top of the wing. When the air flows through this region of low pressure, it speeds up according to Bernoulli's prinicple.[20] Of course, this does not explain why the air deflects downward."

I think that says basically what I have been posting; air must be deflected downward, and Bernoulli's principle does not eplain why it deflects downward. Hence, since it doesn't explain, why bother invoking it? Of course, energy is conserved, which is the basis for Bernoulli's principle, but -- duh -- so what?

I used to be a meteorologist, and it is natural for me to regard air flows as the result of pressure differences (You know, the eye of the hurricane has the lowest pressure, not the high velocity winds), but Newton's laws apply. For lift, you need to change the velocity of the air (deflect it downward) and the shape of the airfoil, symmetric or not, is designed to do that. If you look at the pressure distribution along the airfoil, the pressure change relates to the curvature of the flow, not the total velocity. For example, at the leading edge, where the velocity increases as the air is deflected upward (and downward on the other side), the pressure on the surface increases. When the curvature switches to allow the air to follow the upper side of the airfoil, the pressure decreases. If it did not, why would the air follow the contour of the airfoil? When it straightens out toward the trailing edge (still at high speed), the pressure increases. Since the flow is not confined, it's not a venturi, the "horizontal" velocity doesn't change much; there is little shear between the flow following the surface of wing and the rest of the air flowing past it. However, at the very surface of the wing, the air "sticks" to the surface (boundary layer), not moving relative to the wing ("zero velocity"), so, golly, if Bernoulli is right, the pressure should be greatest there! It isn't. It's the same pressure as the air flowing past it.

Bernoulli's principle is fine, elegant physics. My point is that it does not explain why wings generate lift. It also doesn't bear on the question of induced drag and why biplanes are not as efficent as high aspect ratio monoplanes.

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Guru

Join Date: Apr 2007
Posts: 3531
Good Answers: 59
#59
In reply to #56

Re: Re aircraft design

07/24/2009 7:12 AM

That is not what you said, nor what I was objecting to. Your words were "crap" and "a fantasy ...". You know that the effect is present and real in this case. You could accurately have said that "Bernoulli although present has been widely misused in explanations and is best ignored when explaining aircraft lift", and I would not have objected.
Are you aware that you were the first person on this thread to even mention Bernoulli, and that other explanations were already in terms of energy, momentum and interaction???

P.S. What sort of meteorologist?

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Anonymous Poster
#67

Re: Re aircraft design

07/24/2009 1:47 PM

A historical note: during WW-1, Albert Einstein tried to contribute by designing a better wing section, with computations based on the Bernoulli principle. It was a failure. Real world data trumps theoretical analysis.

This topic has stirred up a lot of interest.

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Anonymous Poster
#68

Re: Re aircraft design

07/24/2009 2:24 PM

Final [?] note on topic. Will start a new direction.

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Associate

Join Date: Dec 2007
Posts: 39
Good Answers: 2
#71

Re: Re aircraft design

07/25/2009 10:51 AM

just concentrate on burnalis theorem,


and look the design of airplane. wings just give stability and and help while turning nothing else.........

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Power-User

Join Date: Sep 2008
Posts: 141
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#80

Re: Re aircraft design

11/27/2009 11:18 AM

It depends on the arrangement of the wings and the purpose of the aircraft. Long, high-aspect-ratio wings provide less induced drag and also a lower interference drag component. They do not work well for applications requiring high manueverability or high G-loads. In short, high lift is not the only consideration.

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